Re: [PATCH v9 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests

[Madhavan Srinivasan]

On Sat, 08 Aug 2026 21:41:44 +0530, Amit Machhiwal wrote:
> On POWER systems, newer processor generations can operate in compatibility
> modes corresponding to earlier generations (e.g., a Power11 system running
> in Power10 compatibility mode). In such cases, the effective CPU level
> exposed to guests differs from the physical processor generation.
> 
> This creates a problem for nested virtualization. When booting a nested KVM
> guest (L2) inside a host KVM guest (L1) running in a compatibility mode,
> userspace (e.g., QEMU) may derive the CPU model from the raw hardware PVR
> and attempt to configure the nested guest accordingly. However, the L1
> partition is constrained by the compatibility level negotiated with the
> hypervisor (L0), and requests exceeding that level are rejected, leading to
> guest boot failures such as:
> 
> [...]

Applied to powerpc/next.

[1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
      https://git.kernel.org/powerpc/c/ac3e65ddddf3128c1f0c2187889fab458fcf4a70
[2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM
      https://git.kernel.org/powerpc/c/b76fb087efcdaf6eb5c2c6c5e53c763640fc4b02
[3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV
      https://git.kernel.org/powerpc/c/8735048f54ec827102dcbd757a6c1c1e02613fc4
[4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl
      https://git.kernel.org/powerpc/c/c1721e584244ff79a593d6f0ad0405a2e07b765a

cheers